- Email: [email protected]
OQ208, a non-ideal VLBI calibrator of flux density
Chin. Astron. Astrophys. (1993)17/4, A translation of Acta Astron.Sin. (1993)34/l, 12-16
OQ208,
373-376
@ Pergatnon Press Ltd Printed in Great Britain 0275-1062/93$24.00+.00
a non-ideal VLBI calibrator of flux density t ZHANG
1. Shanghai Observatory, 2. Purple Mountain
Fu-jun’
ZHU Han-shu 2
Chinese Academy Observatory,
of Sciences,
Chinese
Academy
Shanghai
200030,
of Sciences
Abstract OQ208 is used ore and more frequently as a VLBI calibrator of flux density. A VLBI image of 0Q208 at 5 GHz is firstly provided in this paper. Its structure consists of a compact core and a component. The angular distance between the core and the component is about 7 milli-arcsecond; the position angle is -125 degrees. In principle, OQ208 is not an ideal VLBI calibrator of flux density. This paper provides the structural information for correcting the structure effect when OQ208 is to be used as a calibrator. Key
words:
Galaxies
nuclei - OQ208 - radio continuum
OQ208 has been optically identified as a Seyfert galaxy. Its magnitude m, = 15.4. The redshift z obtained is 0.0771’1. 0Q208 is un-resolved when it is observed with VLA at both 1.5 GHz and 4.9 GHz121. A very weak extension of emission surrounding the core could be detected with WSRT at 0.9 GHz131. 0Q208 has been used as a calibrator of flux density in some VLBI observations of low frequency or on some short baselines of VLBI experiments14-61. It was observed by Chartlot with VLBI technique at both 2.3 GHz and 8.4 GHz~~. At 2.3 GHz, th e result shows that there is a little extension at the direction of -8”. At 8.4 GHz, 0Q208 exhibits the morphology of double components. the distance between two components is 1.3 milli-arcsecond (mas); the position angle (PA) equals -15”lrl. It was observed at 5 GHz by Zensus et al.lsl and Preuss & Fosbury151. The ratio of the correlated flux density to the total flux density were 0.75 on the baseline between NRAO/Green Bank and NRAO/Green and OVRO lsl, 0.41l”l or 0.561’1 on the baseline between Bonn/Effelsberg Bank, and 0.32 on the baseline between Bonn/Effelsberg and OVROlsl. These results point out clearly that OQ208 can be resolved on intercontinental baselines. Radio source 3C286 was observed selected as a calibrator of flux density
with a global VLBI array at 5 GHz. OQ208 was and fringe finder in that experiment. The telescopes
t Supported by National Natural Science Foundation Received December 15, 1991; revised version October 15, 1992
374
ZHANG Fu-jun & ZHU Han-shu
Table No.
1 Shtiom
Parameters
and Coordinates 0
S.T.
(m)
(K)
of Antennas
Coordinale (m)
s W/JY)
used for VLBI at 5GHz
x
Y
Note
z
used are listed in Table 1. The 32m telescope of Cambridge had just been finished and was the first time to be used in VLBI observation at 5 GHz. Its recorded data was sent to Max-Planck-Institute for Radioastronomy immediately after the experiment had been finished. We searched fringe on the baseline between Cambridge and Bonn/Effelsberg. The result proved that the experiment on this baseline was very successful. 27 x 25m telescopes of VLA formed a phased array (or tied array), which likes a sinle dish with a equivalent aperture of 135 metre, to take part in this observation. After the correlation had been completed at California Institute of Technology, U.S.A., the coefficients of correlation for each baseline were transfered to the correlated flux density with the standard method published in Cohen’s paper 191. OQ208 was used as a calibrator. The total flux density at 5 GHz was 2.75 Jy measured by the 1OOm telescope located at Effelsberg. When we checked the correlated flux density of OQ208, we found that this source was resolved on intercontinental baselines and part of American continental baselines. The correlated flux desities were - 1.8 Jy on the baselines between the telescopes of European VLBI Network (EVN, except the 22m telescope located in Crimea, Russia) and Haystack, - 1.5Jy on the baselines between EVN and NRAO/Green Bank, and - l.lJy on the baselines between EVN and OVRO. 0Q208 was also used as a calibrator in a global VLBI observation at 8.4 GHz (Kus, private communication). They found that it was very difficult to do calibration of flux density with OQ208. This source is resolved seriously at 8.4 GHz. 0Q208 was observed twice in our experiment. The continued time of each scan was half hour. The time span between two scan was six hours. In addition there were quite a few stations included in the global VLBI array. The u-v coverage (shown in Figure 1) is quite good for mapping a source if its structure is not too complex. When we did calibration of flux density, we supposed that 0Q208 is unresolved on the baselines of EVN and the correlated amplitudes were equal on the baseline including Effelsberg and WSRT respectively. During mapping, we edited data several times for improving the quality and the dynamic range of the final map. The data on the baselines including Shanghai 25m telescope and part of baselines including Crimea 22m telescope were deleted. Also, the data with low quality on
OQ 208
Fig. 1
u-v coverage
Fig. 2 The 5 GRz VLBI image of OQZO8 Restoring beam: 3 x 1 mas’, PA = 0’. Peak flux: 2.23 .Jy/beam. Contour levels: (-0.5,0.5,1.0,2.0,4.0,8.0,16.0,32.0,64.0,90.0) % x2.23 Jy/beam
375
376
ZHANG Fu-jun & ZHU Han-shu
some baselines were deleted. Whole procedure of mapping was completed with AIPS. Phase self calibration was executed at first. After the phase remained stable, we did amplitude self calibration. The length of solution used in the last amplitude self calibraiton was 60 minutes. The final result is shown in Figure 2. The restoring beam is 1 x 3mas2, PA = O”. The structure of OQ208 at 5 GHz consists of a compact core and a component. The distance between the core and the component is N 7 mas; the position angle PA of the component is about -125’. The density ratio of the core to the component is 13:l. Comparing our result with ones obtained at both frequencies of 2.3 GHz and 8.4 GHz, we find that the position angle of the component changes with the observing frequency. It is possible that the frequency effect plays an important role for the structure of OQ208 . There is not much information yet on the structure of OQ208 for lacking of systematic VLBI observation. But this source is used more and more frequently as a VLBI calibrator of flux density, at the task of bringing in some error caused by its structural effect. The structure information of 0Q208 is provided for the first time with high dynamic range from our 5 GHz VLBI observation. We conclude that 0Q208 is not an ideal calibrator of flux density in global VLBi observation, specially at high observing frequency. The structural effect should be considered when OQ208 is to be used as a calibrator of flux density. The correction could be done with the information of its structure provided in the paper. ACKNOWLEDGMENT ZHANG Fu-jun acknowledges the support by Professor R. Booth with financial aid for his visit at Onsala Space Observatory. We thank Dr. A. Kus who suggested us to do mapping for OQ208 with our VLBI data at 5GHz.
References
[ll [21 [31
Blake, G. B., Argue, A. N. and Kenworthy, C. M., ApJ, 1970, 160, L67 Perley, Ft. A., FormaIont, E. B. and Johnson, K. J., ApJ, 1982, 255, L93 van der Lann, H., Zieba, S. and Noordam, J. E., In: IAU Symp. 110, VLBI and Compact Radio Sources, ed. R. Fanti, K. Kellermann and G. Setti (Dordrecht: Reidel), 1984, p.9 Jones, D. L., Sramek, R. A. and Terzian, Y., ApJ, 1981,247,
L57
Preuss, E. and Fosbury, R. A. E., MNRAS, 1983,204,783 Wu, S. Y., PauIiny-Toth, I. I. K. and Porcas, Ft. W., CAA 1987 11/3,201= Charlot, P., A&A, 1990,229,51 Zensus, J. A., Porcas, R. W. and PauIiny-Toth, 1.1. K., A&A, 1984,133,27 Cohen, M. H., ApJ, 1975,201,249
AApS 1987 7/2,113
OQ208,
373-376
@ Pergatnon Press Ltd Printed in Great Britain 0275-1062/93$24.00+.00
a non-ideal VLBI calibrator of flux density t ZHANG
1. Shanghai Observatory, 2. Purple Mountain
Fu-jun’
ZHU Han-shu 2
Chinese Academy Observatory,
of Sciences,
Chinese
Academy
Shanghai
200030,
of Sciences
Abstract OQ208 is used ore and more frequently as a VLBI calibrator of flux density. A VLBI image of 0Q208 at 5 GHz is firstly provided in this paper. Its structure consists of a compact core and a component. The angular distance between the core and the component is about 7 milli-arcsecond; the position angle is -125 degrees. In principle, OQ208 is not an ideal VLBI calibrator of flux density. This paper provides the structural information for correcting the structure effect when OQ208 is to be used as a calibrator. Key
words:
Galaxies
nuclei - OQ208 - radio continuum
OQ208 has been optically identified as a Seyfert galaxy. Its magnitude m, = 15.4. The redshift z obtained is 0.0771’1. 0Q208 is un-resolved when it is observed with VLA at both 1.5 GHz and 4.9 GHz121. A very weak extension of emission surrounding the core could be detected with WSRT at 0.9 GHz131. 0Q208 has been used as a calibrator of flux density in some VLBI observations of low frequency or on some short baselines of VLBI experiments14-61. It was observed by Chartlot with VLBI technique at both 2.3 GHz and 8.4 GHz~~. At 2.3 GHz, th e result shows that there is a little extension at the direction of -8”. At 8.4 GHz, 0Q208 exhibits the morphology of double components. the distance between two components is 1.3 milli-arcsecond (mas); the position angle (PA) equals -15”lrl. It was observed at 5 GHz by Zensus et al.lsl and Preuss & Fosbury151. The ratio of the correlated flux density to the total flux density were 0.75 on the baseline between NRAO/Green Bank and NRAO/Green and OVRO lsl, 0.41l”l or 0.561’1 on the baseline between Bonn/Effelsberg Bank, and 0.32 on the baseline between Bonn/Effelsberg and OVROlsl. These results point out clearly that OQ208 can be resolved on intercontinental baselines. Radio source 3C286 was observed selected as a calibrator of flux density
with a global VLBI array at 5 GHz. OQ208 was and fringe finder in that experiment. The telescopes
t Supported by National Natural Science Foundation Received December 15, 1991; revised version October 15, 1992
374
ZHANG Fu-jun & ZHU Han-shu
Table No.
1 Shtiom
Parameters
and Coordinates 0
S.T.
(m)
(K)
of Antennas
Coordinale (m)
s W/JY)
used for VLBI at 5GHz
x
Y
Note
z
used are listed in Table 1. The 32m telescope of Cambridge had just been finished and was the first time to be used in VLBI observation at 5 GHz. Its recorded data was sent to Max-Planck-Institute for Radioastronomy immediately after the experiment had been finished. We searched fringe on the baseline between Cambridge and Bonn/Effelsberg. The result proved that the experiment on this baseline was very successful. 27 x 25m telescopes of VLA formed a phased array (or tied array), which likes a sinle dish with a equivalent aperture of 135 metre, to take part in this observation. After the correlation had been completed at California Institute of Technology, U.S.A., the coefficients of correlation for each baseline were transfered to the correlated flux density with the standard method published in Cohen’s paper 191. OQ208 was used as a calibrator. The total flux density at 5 GHz was 2.75 Jy measured by the 1OOm telescope located at Effelsberg. When we checked the correlated flux density of OQ208, we found that this source was resolved on intercontinental baselines and part of American continental baselines. The correlated flux desities were - 1.8 Jy on the baselines between the telescopes of European VLBI Network (EVN, except the 22m telescope located in Crimea, Russia) and Haystack, - 1.5Jy on the baselines between EVN and NRAO/Green Bank, and - l.lJy on the baselines between EVN and OVRO. 0Q208 was also used as a calibrator in a global VLBI observation at 8.4 GHz (Kus, private communication). They found that it was very difficult to do calibration of flux density with OQ208. This source is resolved seriously at 8.4 GHz. 0Q208 was observed twice in our experiment. The continued time of each scan was half hour. The time span between two scan was six hours. In addition there were quite a few stations included in the global VLBI array. The u-v coverage (shown in Figure 1) is quite good for mapping a source if its structure is not too complex. When we did calibration of flux density, we supposed that 0Q208 is unresolved on the baselines of EVN and the correlated amplitudes were equal on the baseline including Effelsberg and WSRT respectively. During mapping, we edited data several times for improving the quality and the dynamic range of the final map. The data on the baselines including Shanghai 25m telescope and part of baselines including Crimea 22m telescope were deleted. Also, the data with low quality on
OQ 208
Fig. 1
u-v coverage
Fig. 2 The 5 GRz VLBI image of OQZO8 Restoring beam: 3 x 1 mas’, PA = 0’. Peak flux: 2.23 .Jy/beam. Contour levels: (-0.5,0.5,1.0,2.0,4.0,8.0,16.0,32.0,64.0,90.0) % x2.23 Jy/beam
375
376
ZHANG Fu-jun & ZHU Han-shu
some baselines were deleted. Whole procedure of mapping was completed with AIPS. Phase self calibration was executed at first. After the phase remained stable, we did amplitude self calibration. The length of solution used in the last amplitude self calibraiton was 60 minutes. The final result is shown in Figure 2. The restoring beam is 1 x 3mas2, PA = O”. The structure of OQ208 at 5 GHz consists of a compact core and a component. The distance between the core and the component is N 7 mas; the position angle PA of the component is about -125’. The density ratio of the core to the component is 13:l. Comparing our result with ones obtained at both frequencies of 2.3 GHz and 8.4 GHz, we find that the position angle of the component changes with the observing frequency. It is possible that the frequency effect plays an important role for the structure of OQ208 . There is not much information yet on the structure of OQ208 for lacking of systematic VLBI observation. But this source is used more and more frequently as a VLBI calibrator of flux density, at the task of bringing in some error caused by its structural effect. The structure information of 0Q208 is provided for the first time with high dynamic range from our 5 GHz VLBI observation. We conclude that 0Q208 is not an ideal calibrator of flux density in global VLBi observation, specially at high observing frequency. The structural effect should be considered when OQ208 is to be used as a calibrator of flux density. The correction could be done with the information of its structure provided in the paper. ACKNOWLEDGMENT ZHANG Fu-jun acknowledges the support by Professor R. Booth with financial aid for his visit at Onsala Space Observatory. We thank Dr. A. Kus who suggested us to do mapping for OQ208 with our VLBI data at 5GHz.
References
[ll [21 [31
Blake, G. B., Argue, A. N. and Kenworthy, C. M., ApJ, 1970, 160, L67 Perley, Ft. A., FormaIont, E. B. and Johnson, K. J., ApJ, 1982, 255, L93 van der Lann, H., Zieba, S. and Noordam, J. E., In: IAU Symp. 110, VLBI and Compact Radio Sources, ed. R. Fanti, K. Kellermann and G. Setti (Dordrecht: Reidel), 1984, p.9 Jones, D. L., Sramek, R. A. and Terzian, Y., ApJ, 1981,247,
L57
Preuss, E. and Fosbury, R. A. E., MNRAS, 1983,204,783 Wu, S. Y., PauIiny-Toth, I. I. K. and Porcas, Ft. W., CAA 1987 11/3,201= Charlot, P., A&A, 1990,229,51 Zensus, J. A., Porcas, R. W. and PauIiny-Toth, 1.1. K., A&A, 1984,133,27 Cohen, M. H., ApJ, 1975,201,249
AApS 1987 7/2,113